I consider irresponsible and unwise NOT to document something that is going to change. In particular, when I change something I need to know the interface with

I consider irresponsible and unwise NOT to document something that is going to change. In particular, when I change something I need to know the interface with detail, the input and the output and EXACTLY what the black box is expected to do, special cases included. I just cant tell you how much time I have wasted in my life because the original programmer FORGOT the interface, or its details.(Hey, I made this 6 month ago, Do you expect me to remember the details?) I couldn't care less about the blackbox if we are going to change it, but its behavior needs to be documented.